In the magneto-optic recording process, a vertically magnetizable recording layer is initially sensitized by simultaneously subjecting it to a uniform magnetic field and a temperature which exceeds its Curie temperature (e.g., 400 degrees C.). The magnetic field, being directed perpendicular to the recording layer, serves to uniformly align all of the magnetic domains therewith. Once all the magnetic domains are facing in the same direction, the recording layer is ready to record information. Such recording is effected by subjecting the recording layer to a magnetic field of reverse polarity while scanning the layer with an intensity-modulated laser beam.
During the recording process, a laser beam intensity is switched between high and low levels, representing the digital (binary) information being recorded. Only the high level is sufficiently intense to raise the temperature of the irradiated portion of the recording layer to above its Curie temperature; thus, digital information is recorded at the point of incidence of the laser as the more intensely irradiated magnetic domains flip in orientation to align themselves with the reverse magnetic bias field. Playback of the recorded information is commonly achieved by scanning the information tracks with a plane-polarized beam of radiation and monitoring the reflected beam for shifts in the plane of polarization, as produced by the well known Kerr effect. To erase the recorded information, the polarity of the applied external magnetic field is reversed, and the recording layer is scanned with a beam of sufficient intensity to again heat the recording layer to above its Curie temperature. After this erasure step, all of the irradiated magnetic domains will again face in the same direction.
Various schemes have been proposed to achieve the magnetic field inversions required in switching between the record and erase modes of the magneto-optic recording process. In the disclosures of U.S. Pat. Nos. 5,020,042 and 5,291,345, for example, the field inversion apparatus consists of a magnetic field producing coil surrounding the magnet. When the coil is energized, the field that it creates imparts a torque to the magnet forcing it to rotate.
Although the presently known and utilized apparatus and method for rotating the magnet are satisfactorily, they are not without drawbacks. First, the magnetic coupling between the magnet and the coil is inefficient and requires substantial power. Secondly, the magnet field from the coil permeates the surrounding region and can give rise to undesired electromagnetic interference with neighboring electrical components thereby degrading their performance.
Consequently, a need exists for an apparatus for selectively inverting an external magnetic field in a magneto-optic recording system so that electromagnetic interference is eliminated and less power is consumed.